Polyols suitable for production of polyurethane materials such as flexible or rigid foams or solid materials such as elastomers are generally obtained by polymerization of suitable alkylene oxides onto polyfunctional starter compounds, i.e. those containing a plurality of Zerewitinoff-active hydrogen atoms. For the performance of these polymerization reactions, a wide variety of different processes have been known for some time, some of which are complementary to one another:
A first method of industrial significance is the base-catalyzed addition of alkylene oxides onto starter compounds having Zerewitinoff-active hydrogen atoms, and another is the frequent use of double metal cyanide compounds (“DMC catalysts”) for the performance of this reaction. With the use of highly active DMC catalysts described, for example, in U.S. Pat. No. 5,470,813, EP-A 700 949, EP-A 743 093, EP-A 761 708, WO 97/40086, WO 98/16310 and WO 00/47649, polyether polyol preparation is possible at very low catalyst concentrations (25 ppm or less), and so it is no longer necessary to remove the catalyst from the finished product. However, these catalysts are unsuitable for the preparation of short-chain polyols or of polyols having a high content of oxyethylene units, especially those having end blocks rich in oxyethylene units.
Basic catalysts which have long been known, for example those based on alkali metal hydroxides, allow problem-free preparation of short-chain polyols and/or of polyols having a high content of oxyethylene units; in that case, the catalyst generally has to be removed from the alkaline crude polymer by means of a separate workup step. The (Lewis) acid-catalyzed addition of alkylene oxides onto suitable starter compounds is of minor importance.
The base-catalyzed addition of alkylene oxides, for example ethylene oxide or propylene oxide, onto starter compounds having Zerewitinoff-active hydrogen atoms, as already mentioned, is effected in the presence of alkali metal hydroxides, but it is also possible to use alkali metal hydrides, alkali metal carboxylates or tertiary amines, for example N,N-dimethylbenzylamine or N,N-dimethylaminoethanol, or aromatic amines, for example of the imidazole type, or derivatives thereof. In the case of amine-catalyzed alkylene oxide addition reactions, it is possible to dispense with a further workup, provided that the presence of the amines in these polyols does not impair the subsequent production of polyurethane materials. However, amine catalysis allows only the preparation of polyols having comparatively low equivalent weights; see, for example, Ionescu et al. in “Advances in Urethane Science & Technology”, 1998, 14, p. 151-218.
After the alkylene oxides have been added on with use of basic catalysts, for example alkali metal hydroxides, alkali metal hydrides or alkali metal carboxylates, the polymerization-active sites on the polyether chains have to be deactivated. Various procedures are possible for this purpose. For example, it is possible to neutralize with dilute mineral acids such as sulfuric acid or phosphoric acid, or with (hydroxy)carboxylic acids. Optionally, the actual neutralization step may be preceded by a hydrolysis step. The strength of the second dissociation stage of sulfuric acid is sufficient to protonate the alkali metal hydroxides formed as a result of hydrolysis of the active alkoxide groups, such that 2 mol of alkoxide groups can be neutralized per mol of sulfuric acid used. Phosphoric acid, in contrast, has to be used in an equimolar amount to the alkoxide groups to be neutralized.
In many cases, downstream polyurethane applications do not tolerate the dissolved salt content (for example after neutralization with hydroxycarboxylic acids), or the visual appearance of the cloudy polyols containing the undissolved salts which is obtained after neutralization with inorganic mineral acids is perceived to be detrimental to quality. Accordingly, the salts formed generally have to be removed. The distillation and filtration processes performable in a particularly inexpensive manner for this purpose frequently do not have good reproducibility, meaning that the salts sometimes occur in a quality of poor filterability, which can entail time-consuming and repeated filtration and recrystallization processes.
Specifically for the case of neutralization of alkaline crude polyols with inorganic acids, especially phosphoric acid, processes having reproducible formation of the salt in a quality of good filterability have been developed. For example, De Lucas et. al. in Organic Process Research & Development 1999, 3, p. 166-171, in the case of neutralization with phosphoric acid, are concerned with the parameters of stirrer speed, ratio of neutralization acid/alkali metal hydroxide (called the “neutralization level”), water/polyol ratio and water evaporation rate. EP 1292631 optimizes the amount of water used in the neutralization, in order to avoid caking of salt within the neutralization tank and hence also to maintain good heat transfer values over prolonged campaign durations. In this patent application, in example 2, stirring times of duration 75 min after addition of the acid to the alkaline crude polyol are specified; however, no statement is made as to the “contacting time” of acid and alkaline polyol, i.e. as to the period of time from the commencement to the conclusion of the complete addition of the acid to the crude polyol. With a comparable aim, protonatable or protonated nitrogen compounds are added as crystallization aids to neutralized polyether polyols prior to the distillative removal of water in DE 10250429. Better crystal formation and hence improved filtration characteristics are achieved in EP 1517941 by the addition of alkaline compounds during the distillation of the neutralized polyether polyol. According to the teaching of WO 9947582, low-odor polyethers are obtained by over-neutralization of alkaline crude polyethers in the presence of water of hydrolysis, followed by the neutralization of the acid excess with base after a certain stirring time. In this application too, “contact times” of 10 min up to 5 h are specified (page 5 lines 9-11, examples 1 and 2). However, these “contact times” should at best be understood as being the sum total of a “contacting time” of acid and alkaline polyol and a subsequent stirring time. It is likewise the case that no specific statement is made as to the period of time from the commencement up to the conclusion of the complete addition of the acid to the crude polyol. According to the teaching of WO 2010145899, a particular combination of neutralization level and water content prior to filtration facilitates the removal of salts and gives the polyether polyol in a simple manner with the specified acid content. According to the teaching of U.S. Pat. No. 4,507,475, reproducible filtration results and low-odor polyethers are obtained by neutralization of the crude polyol with phosphoric acid in the presence of small amounts of water, the addition of adsorbents, and removal of salts prior to the distillative removal of water.
Against the background of the prior art, it is found, however, that there is still a need for optimization in relation to a simple and reproducible process for workup of alkaline crude polyols, especially with regard to the obtaining of non-cloudy products having low residual salt contents and the avoidance of protracted filtrations which have to be repeated in some cases.